The invention pertains to a process for enhancing the melt strength of a polypropylene by mixing a peroxydicarbonate with the polypropylene and heating the polypropylene/peroxydicarbonate mixture.
Such a process is known from EP-B-0 384 431. This reference describes a process for preparing polypropylene (PP) with a branching index of less than 1 comprising (1) mixing a low decomposition temperature peroxide, such as a peroxydicarbonate, with the PP, (2) heating or maintaining the resulting mixture at a temperature from room temperature up to 120.degree. C., and then (3) heating the PP at a temperature of 130 to 150.degree. C. to deactivate substantially all the free radicals present in said PP. The obtained PP has a significant amount of long chain branches and has an increased weight average molecular weight giving the polymer a significant strain hardening elongational viscosity. It is mentioned that in step (3) the heating may be done by extrusion or in a fluidized bed. It is further stated that at a temperature above 120.degree. C. an essentially linear polymer with little or no branching is obtained.
Apparently, the significant strain hardening elongational viscosity is related to an improved melt strength of the obtained PP due to long chain branching. It is only mentioned in this publication that step (3) of this process can be performed by extrusion, in the examples the entire process is carried out in a sealed reaction vessel. A disadvantage of this process is that it comprises three steps, which is undesirable in practice.
DE-A-4340194 (U.S. Pat. No. 5,416,169) describes a process for preparing PP having a high melt strength and a chain branching coefficient of 1, by mixing bis(2-ethylhexyl)peroxydicarbonate with a linear, crystalline PP, followed by heating at 70 to 150.degree. C. In a subsequent step, the PP is taken out of the reaction vessel and is melt-kneaded. The peroxydicarbonate may be dissolved in an inert solvent before it is added to the PP. It is stated that other peroxydicarbonates, having a similar decomposition temperature, cannot be used for this purpose.
A disadvantage of this process is that it can only be performed using one specific peroxide, i.e., bis(2-ethylhexyl)peroxydicarbonate. Another drawback is that the process is performed in two steps, which is undesirable and uneconomical in practice.
It is the object of the present invention to provide a process which does not have the above-mentioned drawbacks and which process yields PP having a good melt strength.